1. Field of the Invention
The present invention relates to a carrier for a two-component type electrophotographic developer for use in a copying machine, printer or the like, and a developer using said carrier.
2. Prior Art
A two-component type developer used for electrophotography is composed of a toner and a carrier. The carrier is stirred and mixed with the toner in a development box to give a desired charge to the toner, and then carries the thus-charged toner onto electrostatic latent images on a photoreceptor to develop the latent images, thereby forming toner images.
The carrier thus used remains on a magnet, and is then returned again to the development box, stirred again and mixed with a fresh toner for repeated use.
Accordingly, it is a matter of course in order to make it possible to stably keep desired image characteristics (such as an image density, fog, white spots (or carrier scattering), gradation, resolution) from the initiation of service life test until the end that the carrier constituting the developer is required to exhibit stable constant characteristics during the period of service life.
Conventional carriers for an electrophotographic developer include reduced iron powder, atomized iron powder, iron powder prepared by pulverizing cutting wastage and subjecting the obtained particles to size classification, and surface-oxidized iron powder having a thin iron oxide layer on the surface. However, these conductive carriers have too low resistance and even firmly surface-oxidized iron powder exhibits a dielectric breakdown voltage of as low as 300 V or below, though it is most excellent in breakdown strength among them. Therefore, when a low bias voltage is applied in the development using such a carrier, leakage occurs, so that the solid black image area thus developed has a high density but is not uniform, and the resulting copy has image deficiencies such as many brush marks and distortion of fine-linear images.
Further, various resin-coated iron carriers obtained by coating the surface of iron powder with various resin have also been known (see Japanese Patent Application Laid-Open Gazettes Nos. Sho 56-50337 and Sho 56-84402).
When the core shape off the resin-coated iron carrier is not uniform, the resin peels off from the carrier core material during the service life test to result in leakage phenomenon at the development because of the low resistance of the core material.
On the other hand, in a spherical iron powder particle (spherical steel particle), which is easy to coat a resin uniformly, as the core, the electric field for development in a solid black area is weakened by the injection of charge from a magnet roll in the initial image of development owing to the insulating properties of the carrier, so that the solid black image developed has a lowered density particularly in the central area of the image, i.e., suffers from so-called edge effect.
The spherical steel particle has a large true specific gravity (about 7.8) and an apparent density of 4.5 to 5.0 g/cm.sup.3, so that toner particles fusion-adhere to the surface of the carrier particles during the service-life test owing to the friction and/or collision of carrier particles with each other to cause the "spent"-phenomenon and that the resin layer peels off significantly to expose the conductive core, which causes leakage to and the initial image qualities are not maintained. Thus, no satisfactory durability has been attained as yet with respect to the resin-coated carrier having a spherical steel particle as the core.
There has recently been proposed the use of a soft ferrite represented by the formula: MO.sub.a M'O.sub.b (Fe.sub.2 O.sub.3).sub.x (wherein M and M' each represents a metal element; and a, b and x are each an integer), for example, Ni--Zn ferrite, Mn--Zn ferrite or Cu--Zn ferrite in the carrier used in a two-component type developer system instead of the above surface-oxidized iron powder or resin-coated iron powder according to the prior art for the purpose of overcoming the above disadvantages to attain high-quality images (see Japanese Patent Application Publication Gazettes Nos. Sho 56-52305 and Sho 62-40705). Such carriers are actually commercially available.
Main reasons why the ferrite carrier is suitable for forming a high-quality image are as follows:
(1) the ferrite carrier has a dielectric breakdown voltage of as high as 1000 V or above, so that no potential of electrostatic latent images formed on a photoreceptor leaks to the carrier in development to give no brush marks, etc., PA1 (2) a ferrite carrier is composed of oxides, so that it does not deteriorate in service and exhibits a long service life, PA1 (3) the above ferrite has a true specific gravity of as low as about 5.0 and an apparent density of as low as 2.5 to 3.0 g/cm.sup.3, though the spherical iron (steel) particle has a true specific gravity of as high as about 7.8 and an apparent density of as high as 4.5 to 5.0 g/cm.sup.3. Therefore, the ferrite carrier causes the "spent"-phenomenon to a small extent due to the friction and/or collision of carrier particles with each other and the resin layer peels off to a small extent as compared with the carrier having a spherical iron core. Actually, a currently commercially available developer exhibits a service life lengthened by at least several times, and PA1 (4) since a soft ferrite has a saturation magnetization of 15 to 80 emu/g which is smaller than that of an ordinary iron particle (180 to 200 emu/g), ears formed on a magnetic brush for development is so soft that the toner images formed on a photoreceptor is abraded to a small extent by the ears of brush to develop images excellent in resolution.
As described above, the soft ferrite carrier has many advantageous characteristics for providing high-quality images as compared with a iron powder carrier.
However, commercially available Ni--Zn and Cu--Zn ferrite carriers are not advantageous in that the resistance of the core material is high. For example, Ni--Zn ferrite particle exhibits a resistance of about 8.0.times.10.sup.9 to 2.0.times.10.sup.11 .OMEGA., when a voltage of 250 V is applied thereto, while Cu--Zn ferrite particle exhibits a resistance of about 5.0.times.10.sup.9 to 5.0.times.10.sup.10 .OMEGA., when a voltage of 250 V is applied thereto.
Accordingly, a desired image density is obtained in a narrow region in the development using such a carrier. Specifically, a carrier prepared by coating a soft ferrite particle with a resin completely uniformly does not develop satisfactory solid black images owing to its high insulating properties, while a soft ferrite carrier coated with a thin resin layer has the problem that the resin layer peels off owing to the friction and/or collision of carrier particles with each other particularly in the service life test and does not maintain the initial image qualities, though the carrier is superior to the iron carrier of the prior art in durability. Further, since the core has a high resistance, solid black images of too high a density are difficult to be developed in the initial stage of the development. Therefore, most of the developers are prepared so as to have a lower amount of charge for the purpose of attaining a desired image density, which causes trouble due to environmental variation such as fogging at high humidity and toner scattering in the service life test.
Recently, a proposal has been made that a resin composition incorporated a conductive material in it is applied to the core material in enhanced thickness so that a carrier is prepared which is improved in durability and exhibits a lowered resistance to give a desired image density in development (see Japanese Patent Application Laid-Open Gazette No. Sho 62-182759). However, this proposal has a problem that the conductive material cannot homogeneously be dispersed in the resin, so that the resulting carrier undergoes resistance variation in the service life test to result in a poor durability.
Recently, digital copying machines and laser beam printers have been spread, and these machines and printers are of reversal development system involving the application of a high bias voltage. Therefore, the carrier to be used in them is required to have a higher dielectric breakdown voltage. Further, the development is required to give high-quality images having a high image density and good gradation. Furthermore, the developer is also required to be maintenance-free for use, i.e., to have such a durability as to permit the use over the machine service life.
To lengthen the service life of a carrier, it is necessary to reduce the weight of a carrier. However, no satisfactory carrier has been found as yet.
Further, severe environmental regulation has recently been made in North America and Europe. With respect to the regulation of waste, for example, heavy metals such as Ni, Cu and Zn are the objects of regulation in, for example, Title 22 of the State Law of California, U.S.A. Some of the ferrite carriers of the prior art are also included in the of regulation, when the metal content is high. In the future, the regulation will become even more severe, so that the development of a carrier free from the heavy metals included among the objects of regulation has been expected.
Meanwhile, a stoichiometric ferrite having a Li.sub.2 O content of 16.7 mol % has been proposed as a Li-based ferrite (see Japanese Patent Application Laid-Open Gazette No. Sho 50-56946). A ferrite containing such a stoichiometric ferrite and having a Li.sub.2 O content lower than 16.7 mol % has such a high true specific gravity and such a high apparent density which are not suitable for a high-durability carrier. Further, this ferrite is nearly equivalent to Ni--Zn and Cu--Zn ferrites in resistance, and does not attain a sufficiently high image density in development at a low electric potential.
Further, the mixing ratio of Li.sub.2 O or Li.sub.2 CO.sub.3 to Fe.sub.2 O.sub.3 is low and these starting materials are very different in true specific gravity, so that a homogeneous dispersion of them in each other is difficult. Therefore, when a developer containing the thus produced Li-based ferrite carrier is used, it is liable to cause the carrier to fluctuate in magnetization per particle, and further to cause the carrier to scatter so that many white spots in development are produced.